In recent years, attempts have been made to apply graphene to a variety of products because of its excellent electric characteristic of high conductivity and its excellent physical characteristics such as sufficient flexibility and high mechanical strength.
Application of graphene to power storage devices such as a lithium secondary battery and a lithium-ion capacitor is one of the attempts. For example, an electrode material can be coated with graphene to improve the conductivity of the electrode material for a lithium secondary battery.
As a method for forming graphene, a method of reducing graphite oxide or graphene oxide in the presence of a base is given. In order to form graphite oxide using the method for forming graphene, a method using sulfuric acid, nitric acid, and potassium chlorate as an oxidizer, a method using sulfuric acid and potassium permanganate as an oxidizer, a method using potassium chlorate and fuming nitric acid as an oxidizer, or the like can be employed (see Patent Document 1).
As a method of forming graphite oxide with the use of sulfuric acid and potassium permanganate as an oxidizer, the modified Hummers method is given. Here, the method of forming graphene by the modified Hummers method will be described with reference to FIG. 14.
Graphite is oxidized using an oxidizer such as potassium permanganate in a solvent; thus, a mixed solution 1 containing graphite oxide is formed. After that, in order to remove the remaining oxidizer in the mixed solution 1, hydrogen peroxide and water are added to the mixed solution 1, and a mixed solution 2 is formed (Step S101). Here, unreacted potassium permanganate is reduced by the hydrogen peroxide and then the reduced potassium permanganate reacts with sulfuric acid, whereby manganese sulfate is formed. Then, the graphite oxide is collected from the mixed solution 2 (Step S102). Then, the collected graphite oxide is washed with an acid solution in order to remove the oxidizer which remains in or is attached to the graphite oxide, and subsequently, the graphite oxide is washed with water (Step S103). Note that the washing step Step S103 is performed repeatedly. After that, the graphite oxide is diluted with a large amount of water and centrifuged, and the graphite oxide from which an acid is separated is collected (Step S104). Then, ultrasonic waves are applied to a mixed solution containing the collected graphite oxide and an oxidized carbon layer in the graphite oxide is separated, so that graphene oxide is formed (Step S105). Then, the graphene oxide is reduced, whereby graphene can be formed (Step S106).
For a method of forming graphene by reducing graphene oxide, heat treatment can be employed.